1. Field of the Invention
The invention relates to a circuit member comprising an operating voltage line, a ground connecting line, and a reverse polarity protection device.
2. Description of the Related Art
Such a circuit member is, for example, provided in integrated switching circuits as they are used, for example, as sensors in the automobile industry (compare in this context the patent application EP 0 648 019 A2 of the same applicant). The on-board voltages in the motor vehicle field are generally predetermined by the battery voltage of 12 volt. In real operation, however, considerable deviations of this value upwardly and downwardly can result. The operating voltages that have to be considered in this context can be between 5 volt and up to 24 volt, or even outside of the limits. By external and/or internal protective measures, such as, for example, internal voltage stabilizing devices and an increased voltage stability of the circuit member by means of an adapted circuit member design as well as use of internal overvoltage protection devices, these high operational voltages are permissible. However, when especially at high operational voltages the contacts for the operating voltage and for the ground are accidentally switched so that at the operating voltage line the ground potential and at the ground connecting line the operating voltage is applied, respectively, the circuit member can be destroyed or damaged, for example, due to a thermal overload by currents that are too great in breakthrough situation, or the transistors can be damaged by too great a stress voltage, if no further protective measures are provided. The circuit member known from EP 0 648 019 A2 is provided with a certain protection against such a reverse polarity in that the reverse polarity current is limited by an internal or external resistor.
It is an object of the present invention to develop the protective device of the aforementioned kind such that even for those circuit members, in which the application of the known aforementioned protective devices is impossible, a protection against reverse polarity is made possible. Furthermore, the further development together with the actual switching circuit should be monolithically integratable, so that external reverse polarity protective devices can be eliminated and overvoltages in the reverse polarity situation are harmless for the entire circuitry.
In accordance with the present invention, this is achieved in that the reverse polarity protective device is a transistor connected within the ground connecting line which on a switching circuit provides a ground connection to an inner switching arrangement. This transistor during normal operation, which is defined by having the operational voltage applied to the operating voltage line and having the ground potential applied to the ground connecting line, is in a conducting state, and in the reverse polarity situation, which is defined by having the operating voltage applied to the ground connecting line and having the ground potential applied to the operating voltage line, is in a non-conducting state, wherein the transistor is able to receive in the reverse polarity situation a high operating voltage.
The principle of the invention is that a transistor is additionally provided on the chip between the inner ground contact (this refers to the conventional ground contact on the chip of the monolithically integrated circuit member) and the contact for the internal ground. During normal operation, the transistor should be conducting and in the reverse polarity situation, i.e., when applying the operating voltage to the ground connecting line and the ground potential to the operating voltage line, should be non-conducting. Accordingly, the erroneously applied operating voltage cannot be applied to the actual switching circuit so that the switching circuit is now protected and cannot be destroyed. In order not to destroy the transistor in the reverse polarity situation even at high voltages, it is designed as a high voltage transistor. A significant advantage of this protected device is that no current will flow in the reverse polarity situation.
For achieving the conducting state and the non-conducting state of the transistor, a further development of the invention provides that a control electrode is coupled with the one operating voltage line or directly connected thereto. Accordingly, the voltage value on the operating voltage line controls directly the function of the transistor.
A preferred embodiment of the invention for a standard CMOS (complementary metal oxide silicon) manufacturing process proposes that the transistor serving as a reverse polarity protective device is embodied as a MOS (metal oxide silicon) transistor with an n-conducting channel. Its source and drain electrodes, as is conventional, are embedded in relatively highly doped regions which are of the opposite conductivity type as the p-conducting substrate. The transistor region which during normal operation, i.e., without reverse polarity, serves as the source region is embedded in a relatively weakly doped tub-shaped n-well which is arranged laterally and below the source region and is of the same conductivity type as the transistor and of the opposite conductivity type as the substrate. During normal operation, i.e., without reverse polarity, a conducting channel is formed between the source and the drain. The transistor is thus conducting and, with a suitably dimensioned design, almost no voltage drop occurs across its drain-source path. In the reverse polarity situation the source becomes the drain and the drain becomes the source, and the transistor is thus switched off by its control electrode so that a conducting channel is no longer present. The applied operating voltage thus will drop essentially across the weakly doped tub-shaped n-well so that on the sensitive gate oxide no critical field strength will occur which could result in damage of the transistor.
According to a further embodiment of the invention the transistor is an n-chanel MOSFET (metal oxide silicon field effect transistor) of the enhancement type wherein advantageously the tub-shaped well for embedding the source region (source being related to the normal operating situation without reverse polarity) is relatively weakly doped, for example, nxe2x88x92-doped. It is also conceivable to provide a further transistor for protection against reverse polarity not only in the ground connecting line but in addition also in the operating voltage line. This transistor is then of a different conducting type, i.e., a p-channel MOSFET.
The signal outputs of the inner circuit arrangement with the correlated transistors are also protected by the aforementioned transistor, disclosed as one embodiment according to the invention, against reverse polarity of the operating voltage. According to a further embodiment for protecting the inner switching arrangement against reverse polarity of the supply connectors, a DMOS transistor (diffused metal oxide silicon), which generally has also the desired high-voltage compatibility, is also suitable when also integratable.